Any discussion of the prior art throughout this specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Published PCT application No WO 03/038469 A1 entitled ‘A Method and Device for Chronologically Synchronizing a Location Network’, the contents of which are incorporated herein by reference, discloses methods and systems for generating precise position determinations for a mobile apparatus using positioning signals transmitted from a synchronized network of ground-based transmitters known as positioning-unit devices. Key to these methods and systems is the so-called Time Lock Loop (TLL) which directly measures and corrects timing errors in each positioning-unit device, thereby establishing and maintaining a network of positioning-unit devices transmitting positioning signals which are chronologically synchronized to the timebase of a designated reference transmitter. Once a given positioning-unit device has been synchronized to the timebase of the designated reference transmitter, it can relay the network timebase to further positioning-unit devices that do not have a clear view of the designated reference transmitter, thereby propagating the timebase through an extended network of positioning-unit devices. The Time Lock Loop is extremely robust once established, with the synchronization being unaffected by perturbations such as temperature and voltage changes.
However the TLL methodology disclosed in WO 03/038469 A1 requires the reference transmitter and the positioning-unit devices to be in known and fixed locations with respect to a reference coordinate system. Generally the reference transmitter and the positioning-unit devices are ground-based, although it is also possible for positioning-unit devices to receive reference positioning signals from satellites in geostationary orbits, such as Wide Area Augmentation System (WAAS) satellites. If a reference transmitter and a positioning-unit device were to move relative to each other a Doppler shift would be imposed on the reference positioning signals received by the positioning-unit device. This Doppler will be indistinguishable from clock drift, because both Doppler and clock drift manifest as frequency changes. Consequently the positioning-unit device would, in attempting to synchronize itself to the reference transmitter, erroneously slew its clock, severely degrading the accuracy of position solutions calculated by a mobile apparatus. Methods and devices for synchronizing a kinematic location network are therefore highly desirable, to enable the calculation of accurate position solutions for a mobile apparatus receiving signals from a network of positioning-unit devices in which one or more of the positioning-unit devices are moving relative to the others.